Thee present invention is generally concerned with the deposition by evaporation of a coating of any kind onto a substrate of any kind.
It is more particularly, but not necessarily exclusively, directed to the situation in which the substrate is an ophthalmic lens.
Whether they are made of mineral, organic or composite materials, we know that it is sometimes necessary to apply to ophthalmic lenses after manufacture a surface treatment to enhance certain characteristics of the lenses or to confer particular characteristics on them.
Examples of this are an anti-reflection treatment and/or a hardening treatment.
The present invention is even more particularly directed to the situation in which such treatment is effected by depositing a coating formed of at least one layer of material and where such deposition is effected by evaporation, to be more precise by evaporation in a vacuum.
The coating is usually a multilayer coating, i.e. a coating involving stacking a plurality of layers on top of each other.
The various layers can be of different materials, the same materials deposited with different thicknesses or the same materials deposited with the same thickness.
In the latter case, the layers are deposited with different evaporation parameters so that they have different physical-chemical properties.
For an anti-reflection treatment, for example, it is standard practice to stack at least six layers using four different materials, namely an attachment layer, four layers conferring the required optical effects, and a hydrophobic layer.
Usually a large number of substrates are treated at the same time and the treatment enclosure employed has a rotatable support inside it with a plurality of locations disposed around its rotation axis each adapted to receive a substrate to be treated, in practice in the general form of a spherical dome, and an emitter source from which the material to be deposited is evaporated.
In practice the emitter source is a support on which the material to be evaporated is placed, for example a crucible or a plate, and to evaporate it the material is heated by the Joule effect, by electron bombardment or by cathode sputtering, for example.
In all cases one problem to be overcome is that, for obvious reasons, the deposits must be uniform on each of the substrates and identical on all of them.
The evaporation cone of a material is in practice not isotropic.
It is not rare, in this case, to observe a difference between the thicknesses of the layers from one substrate to another, depending on the material deposited, the parameters conditioning its evaporation, the geometry of the treatment enclosure and the position of the emitter source within the treatment enclosure.
To minimise, if not eliminate, this difference, which can lead to unwanted disparities in characteristics, and in particular colour, between the substrates so treated, placing a mask with an appropriate configuration between the emitter source and the support carrying the various substrates to be treated is known per se.
This is described in U.S. Pat. Nos. 4,380,212 and 4,449,478, for example.
Determined experimentally, the configuration of the mask is such that its spread is greater near the rotation axis of the support than at a distance from that rotation axis, for example.
In this way a layer of material can be deposited on the various substrates treated with substantially the same thickness on all of them.
However, although the mask can in this way optimise the thickness of one layer, this is unfortunately not so for all the layers.
The single mask employed is then optimised for one of the layers and for the others, where this is technically possible, all that can be done is to operate on other parameters, to modify the shape of the corresponding evaporation cone, for example by subjecting the material to be evaporated to a pre-melt treatment of greater or lesser duration.
The distribution of the thin layers is usually more or less neglected, however.
Thus the overall result obtained is not always totally satisfactory, the various substrates treated at the same time having characteristics which are not strictly identical from one of them to another on leaving the treatment enclosure, depending on their position on the support.
Experiments have been carried out to improve on this situation.
For example, providing two different masks in the treatment enclosure, each individually adapted to depositing two different layers, and which are therefore used one at a time, has been proposed.
However, increasing the number of different masks in line with a greater number of layers to be deposited would quickly become prohibitive.
As in the document xe2x80x9cUniformity Deposition Corrector October 1971xe2x80x9d, IBM Technical Disclosure Bulltein, vol. 14, no. 5, October 1971, page 1572, XP002083820, forming the mask using a mobile covering panel has also been proposed.
However, as previously, the possibilities of adapting a mask of the above kind are limited to only two layers.
A general object of the present invention is an arrangement having an increased capacity for adaptation.
To be more precise, it firstly consists in a mask used to control the deposition by evaporation of a coating of any kind onto a substrate of any kind, the mask being of the kind including at least two separate covering panels one of which is mobile and generally being characterized in that the two covering panels, referred to hereinafter for convenience only as lateral covering panels, are substantially coplanar and, under the control of a common actuator, are mobile continuously between two extreme positions, namely a close together position in which the space between them is minimal and a spread apart position in which that space is maximal; it also consists in any treatment enclosure using a mask of the above kind and the process for obtaining a corresponding multilayer coating.
Using the invention, it is advantageously possible, for each layer to be deposited, to optimise the surface area of the mask by adjusting the surface area thereof as closely as possible.
All that is required is to move the two lateral covering panels of the mask a greater or lesser distance apart.
It is therefore possible to obtain substantially the same thickness for the layer deposited on each of the substrates treated at the same time and to obtain good uniformity of characteristics from layer to layer for all the substrates.
Hit and miss operation on other parameters, such as the pre-melt time, can advantageously be dispensed with.
In outline, the mask in accordance with the invention advantageously constitutes a variable surface area mask and, to form a multilayer coating, it is therefore advantageously possible to modify the surface area of the mask if necessary to deposit each of the layers